In some conveyorized manufacturing lines, the individual workpieces are mounted on a pallet and circulated along the conveyor to various processing location work stops to perform assembly on the workpieces. The amount of time required to complete an operation on a workpiece at one work stop is referred to as a cycle time. A cycle time includes not only the amount of time required to perform a specific operation, but also the time required to move the completed workpiece away from the work stop and increment or index the next workpiece into position. Even a small reduction in a cycle time can result in a significant increase in production capacity. Cycle time reductions are especially relevant in high speed manufacturing lines. Cycle time improvements can also reduce the size requirements of the operation, thereby decreasing capital investment expenditures.
There are many ways to reduce cycle times. One way is to incorporate "walking beams" into the manufacturing line to index palletized workpieces at high speeds. A typical walking beam is a cam-driven, elongated beam having a plurality of engagement members for engaging workpieces or pallets on a conveyor. The beam is designed to quickly index or process large numbers of workpieces simultaneously along the conveyor. Unfortunately, walking beams cause the manufacturing line to become synchronous, less flexible and much more expensive.
Another cycle time reducing option for conveyorized manufacturing lines is simply to increase the speed of the conveyor. The downside is that the pallets begin to impact each other and the work stops within the manufacturing line due to the high speeds. These impacts can cause physical and vibrational damage to the process and the workpieces on the pallets.
Yet another way to reduce cycle times is to incorporate a "pre-stop" at each work stop. A pre-stop is located on the conveyor immediately upstream of and adjacent to a work stop. Pre-stops bring the workpiece as close as possible to the work stop to reduce the cycle time and eliminate the need to synchronize the manufacturing line. The time required to move a workpiece pallet from a pre-stop to a work stop is limited by the coefficient of friction between the pallet and the conveyor to accelerate the pallet up to full speed. An improved apparatus and method for indexing pallets at high speeds is needed.